Development of separation membranes for filtrating and separating a specific gas or the like from a mixture of various kinds of gas or the like has been proceeding from the viewpoint of an environment or energy saving. As such a separation membrane, there have been known polymer films such as a polysulfonic film, a silicon film, a polyamide film, and a polyimide film. However, such a film has a problem in chemical resistance or thermal resistance, for example, easy change in quality and deterioration of the film when an organic solvent is contained in the mixture.
On the other hand, a carbon membrane is excellent in thermal resistance and chemical stability as a separation membrane, and a separation membrane having a porous body and a carbon membrane formed thereon has been known. For example, Japanese Patent No. 3647985 discloses a carbon molecular sieve membrane manufactured by forming a coating layer of silica sol, alumina sol, or the like on a surface of a ceramic porous body and forming a carbon membrane adhering to a surface of the coating layer. Since a large number of pores each having a pore diameter of 1 nm or less are present in the carbon molecular sieve membrane, components having a specific molecular diameter can be separated from a mixture of various kinds of gas having different molecular diameters and refined. In addition, Japanese Patent No. 3698107 discloses a gas separation membrane obtained by eliminating a substituent from an aromatic polyimide film having a substituent (sulfonic acid group) capable of being eliminated by thermal decomposition and heating the aromatic polyimide film so that an imide framework may remain.
As disclosed in Japanese Patent No. 3647985, in a method of impregnating the surface of the porous body (porous substrate) with silica sol and forming a carbon membrane thereon, a pore diameter of the carbon membrane increases due to formation of a sol layer. Therefore, separation performance is improved with respect to some kinds of gas having a molecule diameter of 0.43 nm or more and a relatively high molecular weight as in the case of separating a mixed gas of C3H8 and C3H6. However, it has been found that selectivity deteriorates with respect to a substance having a relatively small molecular weight as in separation of a mixed gas of CO2 and CH4, separation of a mixed gas of N2 and O2, and separation of a mixture of water and EtOH, which are industrially useful. Further, a flux is low by the influence of pressure loss due to silica sol, and the separation performance is lower than a method of forming a carbon membrane directly on a porous substrate as before.
In order to enhance both the selectivity and the flex of separation of a mixture using a general carbon membrane formed of one kind of a precursor, there is a method where a molecular structure of a precursor is changed so as to increase the number of pores which contribute to separation or a thickness of the membrane is further decreased. However, there is a limitation on increase in the number of pores by changing a molecular structure, and defects increase when the membrane thickness is decreased to 1 μm or less. Therefore, selectivity deteriorates.